The present invention relates to eccentric screw pumps and motors, and more particularly, to pumps and motors which have a screw-type motor eccentrically disposed within a stator for relative rotational movement.
Eccentric screw pumps or motors consist of a stator with a helical bore or passage within which a helical rotor rotates. The number of threads of the helical rotor is one less than the number of threads in the bore of the stator. During the rotation of the rotor, the rotor rolls with a positive fit in the threading of the bore. In relation to gears, there is a spiral-toothed pinion, which rolls in a spiral-toothed spur wheel, wherein the number of teeth of the pinion and spur wheel differ by one. During the rotation of the rotor, its longitudinal axis ideally moves on a circular path with the diameter of the circular path corresponding to twice the eccentricity ratio to the stator.
Because both the outer surface of the rotor and the bore in the stator are helical with the same turning direction, approximately banana-shaped hollow spaces or chambers are generated along the rotor. These spaces or chambers advance from one end of the stator in the direction towards the other end during the rotation of the rotor. Each of these banana-shaped chambers is isolated and sealed from the other chambers, which enclose other regions of the stator with other regions of the rotor.
To guarantee a good seal between the individual chambers, the stator has an elastomer coating. The inner wall of the stator, for example, may consist of an elastomer material which is pressed onto the rotor in the region of contact points with the rotor. The resulting relative motion between the stator and the rotor is not a pure rolling motion. Due to the seal between the stator and the rotor, a sliding motion occurs over wide stretches.
If an eccentric screw pump is charged with a compressed medium, it also can be used as an eccentric screw motor. This principle can be applied to underground boring motors (i.e. mud motors). Because eccentric pump motors consist of very few components, they are very narrow in diameter but nevertheless can generate great torque.
The medium that is pumped or used for the drive can contain particles without risking damage to the pump or motor, which is another advantage of eccentric screw pumps and eccentric screw motors. Eccentric screw pumps are used, for example, to convey mortar. Thus, it can be used with a material that contains a high percentage of solid particles.
The operating temperature of an eccentric screw pump or an eccentric screw motor is a function of the flow rate, the ambient temperature, the specific heat of the flowing medium, and the friction between the stator and the rotor. The friction generates heat, which is carried off by the medium. An eccentric screw pump reaches operating temperatures up to 300xc2x0 C. depending on the ambient temperature and its operating efficiency. Therefore, it must handle a temperature jump of up to about 280xc2x0 C. when started from room temperature under normal conditions.
The elastomer coating used in screw pumps and motors commonly consists of a synthetic elastomer or a compound of such material with natural rubber. Both materials exhibit a strong temperature profile, i.e., the coefficient of expansion is relatively high. Thus, the thin width in the stator changes considerably as a function of temperature. At low temperatures, the rotor turns easily in the stator, while at high temperatures, the material of the inner coating can expand to the extent that the rotor is practically stopped. Then, if the rotor is turned externally with the aid of a drive, the teeth tear away the elastomer coating in the bore. The friction losses, which occur within the eccentric screw pump or the eccentric screw motor, also are strongly dependent on temperature and on the medium.
For geometries used until now, the bore of the stator has had a relatively smooth, wave-shaped profile. This wave-shaped profile can be calculated by a person skilled in the art by means of known geometric relationships. In the broadest sense, the teeth have the shape of cycloid teeth, wherein the teeth and the teeth gaps are rounded.
Why the stator may be stopped in the rotor as mentioned above, can be understood when considering a disk-shaped section of the eccentric screw pump, assuming there are five bores in the stator and four teeth on the rotor. In one position, one tooth of the rotor descends into a tooth gap of the bore, while the opposite tooth of the rotor slides over the opposite tooth gap of the bore during the rolling motion. The more the elastomer coating expands inwardly in the radial direction due to temperature expansion, the smaller the distance between the tooth crown and the base of the opposite tooth gap, which correspondingly increases the stopping force of the rotor.
The operating temperature range of known eccentric screw pumps and eccentric screw motors also cannot be increased since the inner dimensions of the elastomer coating are designed according to the high operating temperature. In the cold state, the rotor would no longer be sufficiently sealed relative to the inner wall of the bore since the elastomer coating shrinks too much as a fumction of temperature.
Eccentric screw pumps also are used for the purpose of conveying pure water. Here, water is a relatively good lubricant for the rubber-metal material mating pair. However, due to the frictional movement between rotor and stator inner wall, the water film is stripped, which leads to dry contact between the coating and the rotor over a relatively broad strip, which produces increased squeaking noises.
It is an object of the present invention to provide an improved eccentric screw pump or motor which is functional over a broader temperature range.
Another object is to provide an eccentric screw pump or motor as characterized above which incurs less internal friction at a given temperature than prior art designs.
A further object is to provide an eccentric screw pump or motor which is operable for use with pure water with lesser tendency to generate noise.
For displacement-type machines according to the invention, the concept behind the profile of the inner bore of the stator of the inventive machine first will be described in relation to eccentric screw pumps or motors according to the state of the art. In groove profiles resulting from the concept of the invention, there are flat grooves, which transition with rounded side surfaces into the other profile. In this case, the profile of the inner bore is similar to waves set one next to another, which are separated from each other by the grooves. Such grooves can be placed in the crown surfaces of the teeth or in the thread valleys of the inner bore of the stator or both in the crown surfaces of the teeth and also in the teeth gaps. As a result of these grooves, the lengths over which the rotor contacts the coating in a frictionally engaged way, as viewed in the circumferential direction, are significantly reduced for the same sealing effect. Simultaneously, the contact force can be decreased.
As soon as a rotor tooth bridges a groove, two seal edges are produced which seal the tooth. Each of these seals can be pressed with considerably less force without resulting in breaks in the seal. In addition, the material of the coating can be concentrated in the region of the groove for passage of the tooth of the rotor from the raised region, which achieves greater flexibility.
Even if the width of the inner bore becomes smaller due to thermal expansion of the elastomer material, tolerable contact forces are still produced. The reduction of contact force, as well as the reduction of the thin width, is realized from the ability to displace the material as described above into the region of a groove. Thus, the material is in a better position to expand. In addition to the grooves, there also can be waves on both sides of each groove, which are raised relative to the smooth profile. The configuration of the stator according to the invention is advantageous for eccentric screw arrangements that work both as pumps and as motors.
The jacket that surrounds the elastomer coating can border either a cylindrical inner space or a helical inner space. In the case of the helical inner space, the thickness of the elastomer coating is approximately the same at all points, while for the cylindrical inner space, the thickness in the region of the teeth of the bore is significantly thicker and thus more flexible. There can be additional waves or grooves not only in the crowns of the teeth or in the teeth gaps but also in the sides that connect the crown of the teeth to the teeth gaps.
The dimensions of the waves or grooves, seen in the circumferential direction, can be greater at the crowns of the teeth than in the teeth gaps. Especially favorable relationships are produced if the waves in the teeth are symmetrical to a crown line, which follows the contours of the teeth and which exhibits the smallest radial distance from the bore axis. Thus, directly on the crown line there is no wave. The same structure also can be used in the teeth gaps.
An especially favorable arrangement relative to the tooth gap is produced if there is a wave directly in the valley line, which exhibits the greatest radial distance from the axis of the bore. In this way, the tooth gap in which the tooth of the rotor cuts most strongly can provide an especially soft support. At least for a few waves or grooves, the cross-sectional profile through the waves is essentially symmetrical as seen in the circumferential direction of the bore.
According to the purpose of the application, the pitch of the waves or grooves can equal the pitch of the stator or the pitch of the rotor, or alternativcly,_the pitch can assume an intermediate value. In that regard, differing pitches have special advantages if water is to be pumped or used as the drive medium. The grooves can produce equal lubricant chambers from which water can be discharged for lubrication.